1. Field of the Invention
The present invention relates to a semiconductor integrated circuit. For example, the invention is applied to an output driver circuit or an input receiver circuit, which includes a terminating resistor element.
2. Description of the Related Art
When high-speed data transfer is executed between semiconductor devices, there is a tendency that a problem arises due to reflection of a signal, which is caused by mismatching of impedance between transmission lines on a board (PCB) and input/output pins (or only input pins or only output pins) of semiconductor devices. In recent years, in semiconductor devices which execute high-speed data transfer, such a technique has widely been used that terminating resistor elements, which are formed of polysilicon, P-type or N-type doped silicon or transistors, are mounted at input/output pins (or only input pins or output pins) in semiconductor devices, thereby making impedance matching with characteristic impedance of transmission lines on the PCB.
In the above-described semiconductor device, in order to enhance impedance matching with transmission lines on the PCB, it is necessary to exactly measure and select the resistance value of the terminating resistor in the semiconductor device, and to set the resistance value at a target measurement value.
For this purpose, for example, as disclosed in U.S. Pat. No. 6,924,660, wide use has been made of a technique of measuring the terminal end resistance value during the operation of the system mounted on the PCB, and setting or correcting the resistance value. In this technique, however, there is a disadvantage, as an adverse effect, that the number of circuit elements increases and the device area increases. Furthermore, depending on purposes of use, there are such specifications that the measurement and setting of the terminal end resistance value during normal operation are impossible or difficult. Under the circumstances, in many cases, the measurement of the terminal end resistance is performed at the time of a wafer test in a semiconductor device fabrication process, or at the time of a product test after package sealing, and then the setting (correction) of the terminal end resistance value is performed.
Referring, for example, to the case of a differential output driver including a terminating resistor, the method of measuring the resistance value of the terminating resistor is explained. In this case, the resistance value of the terminating resistor is a series resistance of a P-type MOS transistor (P1) which is used for switching, and a resistor element (R1).
In this case, however, the measured resistance value of the terminating resistor necessarily becomes a value including an additional value of contact resistance between a jig of a tester and a pad. When the semiconductor device, which is sealed in the package, is mounted on the PCB, the respective terminals of the semiconductor device and the PCB are connected by soldering with a very low resistance value, and the contact resistance becomes a negligible value. However, at the time of the test, the contact value is present. In particular, at the time of the wafer test, stable contact at low resistance is difficult.
The above-described contact resistance is a non-negligible value for the terminal end resistance that is to be measured. Besides, the resistance value of this contact resistance varies from time to time in measurement, depending on various conditions such as contamination of the jig, the number of times of measurement, and the contact pressure between the terminal of the semiconductor device and the jig. It is thus impossible to remove such a variation as a fixed offset from the measured resistance value.
If pads for voltage measurement are added to the pads for terminal end power supply and for measurement in the differential output driver, it becomes possible to measure the terminal end resistance by eliminating the influence of the contact resistance.
However, this circuit configuration is not practical since it is necessary to additionally provide the pads for voltage measurement, and thus the area of occupation increases due to the increase of the pin capacitance of the output terminal and the increase of the device area.